This invention relates to modem receiver apparatus in which a modulated carrier signal represents digital bits.
Timing recovery and timing initialization are important aspects of modems.
D. L. Lyon "Envelope-Derived Timing Recovery in QAM and SQAM Systems", IEEE Transactions on Communications, November 1975, pp. 1327-1331 discloses choosing timing phase, in an equalized modem, such that the spectral component at the Nyquist band edge is maximized. This is accomplished by passing an unsampled low-pass demodulated data waveform, first through a narrow-band filter turned to half the baud rate, then to a squaring device, and finally to a narrow-band filtej tuned to the baud rate. 7
Forney, Jr. et al., U.S. Pat. No. 3,978,407 teaches obtaining timing recovery and timing initialization by obtaining an approximation of the absolute value of the difference between two low-pass filtered complex data streams, separated in time by a signaling interval T.
Qureshi et al., U.S. Pat. No. 4,004,226 shows a timing recovery scheme wherein the sampling epoch is altered based upon the difference between the equalizer tap coefficient of greatest magnitude and a desired value for that tap coefficient.
K. Watanabe, K. Inoue and Y. Sato, "A 4800 Bit/s Microprocessor Data Modem", IEEE Transactions on Communications, Vol. COM-26, No. 5, May 1978, pp. 493-498 describes a microprocessor implemented data modem, wherein the sampling timing is dependent upon one of the coefficients of an automatic adaptive equalizer.